1. Field
The disclosed embodiments relate generally to wireless communications, and more specifically to matching the frequency of a received carrier signal in a mobile wireless communication system.
2. Background
As modern-day wireless communication systems become more prevalent, the demand for wireless system capacity increases. In order to support a greater number of subscribers, a wireless service provider can either increase the frequency spectrum used for its systems or find ways to support more subscribers within its already-allocated frequency spectrum. Often unable to acquire additional frequency spectrum, wireless service providers must often look instead for ways to increase capacity without using more spectrum. In other words, wireless service providers must find more efficient ways to use their existing spectrum.
In response to the demand for more efficient use of spectrum, manufacturers of wireless equipment have developed various techniques for increasing the capacity of wireless systems. One way of providing efficient wireless voice and data communications is the use of code division multiple access (CDMA) techniques. Several standards using CDMA techniques have been developed for terrestrial wireless voice and data systems. Examples of such standards include the “TIA/ELA/IS-95 Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System”, hereinafter referred to as “IS-95,” and “TIA/EIA/IS-2000,” hereinafter referred to as “cdma2000.” Additional standards have been proposed for wireless communication systems that are optimized to provide high-speed wireless data communications. Examples of such standards for high-speed wireless data communications include “TIA/EIA/IS-856,” hereinafter referred to as “HDR.”
In an HDR system, the rate at which a user terminal can receive data may be limited by the quality of signals that the user terminal receives. In such a system the data rate of signals transmitted to a user terminal is determined based on measurements of received signal quality made at the user terminal. One type of quality measurement used to determine data rate is the carrier-to-interference (C/I) ratio of the received signal. When the power of the received carrier signal is strong compared to the power of interfering signals, then the C/I value is said to be high. When the power of the received carrier signal is weak compared to the interference, then the C/I is said to be low. When the C/I value is high, the user terminal can receive more data within a given period of time. When the C/I value is low, the rate of data sent to the user terminal is reduced in order to maintain an acceptable frame error rate.
Carrier frequency recovery is one aspect of user terminal design that can greatly affect the C/I perceived by the user terminal. Carrier frequency recovery refers to the generation within a user terminal of a reference carrier signal having the same frequency as a carrier signal received from a base station. The user terminal uses the reference carrier signal to demodulate data signals received from a base station. A mismatch between the reference carrier signal and the received carrier signal, called carrier frequency mismatch, decreases the efficiency of the demodulation process. Such decreased efficiency of demodulation is perceived at the user terminal as a decrease in C/I. Carrier frequency mismatch thus decreases the rate at which data can be sent to the user terminal.
In tension with the need for precise carrier frequency recovery is the desire to minimize the hardware cost of the user terminal. The market for user terminal equipment such as wireless phones and modems is very competitive and is often characterized by low profit margins, or even subsidies by service providers. There is therefore a need in the art for techniques that increase the precision of carrier frequency recovery in user terminal equipment without substantially increasing the cost of user terminal hardware.